This year's cooler temperatures are most likely due to the TOTAL lack of sunspots since spring 2007: Solar Cycle 24 is now 14 months behind schedule, and there have been no sunspots since early December 2007. Cycle 23 slowly ended - though there have been two or three “old cycle” sunspots since November.
No sunspots means very, very little magnetic field from the sun -> so less shielding from the cosmic rays coming in from the galaxy -> so more cosmic rays -> more impacts with the atmosphere -> more cloud nuclei in the mid-atmosphere -> more clouds -> less sunshine getting through + more reflected from the earth -> lower temperatures.
The earth's orbit - though it is interesting to note that there IS 7% less radiation received - has never been discussed w/r global cooling. Particularly, this double-cycle now of the regular summer time longer distance from the sun + the eccentricity might mean this year's summer will be shorter than usual.